Photographic emulsions are comprised of a dispersing medium and silver halide microcrystals, commonly referred to as grains. As the grains are precipitated from an aqueous medium, a peptizer, usually a hydrophilic colloid, is adsorbed to the grain surfaces to prevent the grains from agglomerating. Subsequently binder is added to the emulsion and, after coating, the emulsion is dried. The peptizer and binder are collectively referred to as the photographic vehicle of an emulsion.
Gelatin and gelatin derivatives form both the peptizer and the major portion of the remainder of the vehicle in the overwhelming majority of silver halide photographic elements. An appreciation of gelatin is provided by this description contained in Mees The Theory of the Photographic Process, Revised Ed., Macmillan, 1951, pp. 48 and 49:
Gelatin is pre-eminently a substance with a history; its properties and its future behavior are intimately connected with its past. Gelatin is closely akin to glue. At the dawn of the Christian era, Pliny wrote, "Glue is cooked from the hides of bulls." It is described equally shortly by a present-day writer as "the dried down soup or consomm a of certain animal refuse." The process of glue making is age-old and consists essentially in boiling down hide clippings or ones of cattle and pigs. The filtered soup is allowed to cool and set to a jelly which, when cut and dried on nets, yields sheets of glue or gelatin, according to the selection of stock and the process of manufacture. In the preparation of glue, extraction is continued until the ultimate yield is obtained from the material; in the case of gelatin, however, the extraction is halted earlier and is carried out at lower temperatures, so that certain strongly adhesive but nonjelling constituents of glue are not present in gelatin. Glue is thus distinguished by its adhesive properties; gelatin by its cohesive properties, which favor the formation of strong jellies. PA1 Photographic gelatin is generally made from selected clippings of calf hide and ears as well as cheek pieces and pates. Pigskin is used for the preparation of some gelatin, and larger quantities are made from bone. The actual substance in the skin furnishing the gelatin is collagen. It forms about 35 percent of the coria of fresh cattle hide. The corresponding tissue obtained from bone is termed ossein. The raw materials are selected not only for good structural quality but for freedom from bacterial decomposition. In preparation for the extraction, the dirt with loose flesh and blood is eliminated in a preliminary wash. The hair, fat, and much of the albuminous materials are removed by soaking the stock in limewater containing suspended lime. The free lime continues to rejuvenate the solution and keeps the bath at suitable alkalinity. This operation is followed by deliming with dilute acid, washing, and cooking to extract the gelatin. Several "cooks" are made at increasing temperatures, and usually the products of the last extractions are not employed for photographic gelatin. The crude gelatin solution is filtered, concentrated if necessary, cooled until it sets, cut up, and dried in slices. The residue, after extraction of the gelatin, consists chiefly of elastin and reticulin with some keratin and albumin. PA1 Gelatin may also be made by an acid treatment of the stock without the use of lime. The stock is treated with dilute acid (pH 4.0) for one to two months and then washed thoroughly, and the gelatin is extracted. This gelatin differs in properties from gelatin made by treatment with lime. PA1 Although collagen generally is the preponderant protein constituent in its tissue of origin, it is always associated with various "ground substances" such as noncollagen protein, mucopolysaccharides, polynucleic acid, and lipids. Their more or less complete removal is desirable in the preparation of photographic gelatin. PA1 (1) Photographic silver halide emulsion layers and other layers on photographic elements can contain various colloids alone or in combination as vehicles. Suitable hydrophilic materials include both naturally occurring substances such as proteins, protein derivatives, cellulose derivatives--e.g., cellulose esters, gelatin--e.g., alkali-treated gelatin (pigskin gelatin), gelatin derivatives--e.g., acetylated gelatin, phthalated gelatin and the like, polysaccharides such as dextran, gum arabic, zein, casein, pectin, collagen derivatives, collodion, agar-agar, arrowroot, albumin and the like . . . PA1 Rutenberg et al U.S. Pat. No. 2,989,520; PA1 Meisel U.S. Pat. No. 3,017,294; PA1 Elizer et al U.S. Pat. No. 3,051,700; PA1 Aszolos U.S. Pat. No. 3,077,469; PA1 Elizer et al U.S. Pat. No. 3,136,646; PA1 Barber et al U.S. Pat. No. 3,219,518; PA1 Mazzarella et al U.S. Pat. No. 3,320,080; PA1 Black et al U.S. Pat. No. 3,320,118; PA1 Caesar U.S. Pat. No. 3,243,426; PA1 Kirby U.S. Pat. No. 3,336,292; PA1 Jarowenko U.S. Pat. No. 3,354,034; PA1 Caesar U.S. Pat. No. 3,422,087; PA1 Dishburger et al U.S. Pat. No. 3,467,608; PA1 Beaninga et al U.S. Pat. No. 3,467,647; PA1 Brown et al U.S. Pat. No. 3,671,310; PA1 Cescato U.S. Pat. No. 3,706,584; PA1 Jarowenko et al U.S. Pat. No. 3,737,370; PA1 Jarowenko U.S. Pat. No. 3,770,472; PA1 Moser et al U.S. Pat. No. 3,842,005; PA1 Tessler U.S. Pat. No. 4,060,683; PA1 Rankin et al U.S. Pat. No. 4,127,563; PA1 Huchette et al U.S. Pat. No. 4,613,407; PA1 Blixt et al U.S. Pat. No. 4,964,915; PA1 Tsai et al U.S. Pat. No. 5,227,481; and PA1 Tsai et al U.S. Pat. No. 5,349,089. PA1 Daubendiek et al U.S. Pat. No. 4,414,310; PA1 Abbott et al U.S. Pat. No. 4,425,426; PA1 Wilgus et al U.S. Pat. No. 4,434,226; PA1 Maskasky U.S. Pat. No. 4,435,501; PA1 Kofron et al U.S. Pat. No. 4,439,520; PA1 Solberg et al U.S. Pat. No. 4,433,048; PA1 Evans et al U.S. Pat. No. 4,504,570; PA1 Yamada et al U.S. Pat. No. 4,647,528; PA1 Daubendiek et al U.S. Pat. No. 4,672,027; PA1 Daubendiek et al U.S. Pat. No. 4,693,964; PA1 Sugimoto et al U.S. Pat. No. 4,665,012; PA1 Daubendiek et al U.S. Pat. No. 4,672,027; PA1 Yamada et al U.S. Pat. No. 4,679,745; PA1 Daubendiek et al U.S. Pat. No. 4,693,964; PA1 Maskasky U.S. Pat. No. 4,713,320; PA1 Nottorf U.S. Pat. No. 4,722,886; PA1 Sugimoto U.S. Pat. No. 4,755,456; PA1 Goda U.S. Pat. No. 4,775,617; PA1 Saitouet al U.S. Pat. No. 4,797,354; PA1 Ellis U.S. Pat. No. 4,801,522; PA1 Ikeda et al U.S. Pat. No. 4,806,461; PA1 Ohashi et al U.S. Pat. No. 4,835,095; PA1 Makino et al U.S. Pat. No. 4,835,322; PA1 Daubendiek et al U.S. Pat. No. 4,914,014; PA1 Aida et al U.S. Pat. No. 4,962,015; PA1 Ikeda et al U.S. Pat. No. 4,985,350; PA1 Piggin et al U.S. Pat. No. 5,061,609; PA1 Piggin et al U.S. Pat. No. 5,061,616; PA1 Tsaur et al U.S. Pat. No. 5,147,771; PA1 Tsaur et al U.S. Pat. No. 5,147,772; PA1 Tsaur et al U.S. Pat. No. 5,147,773; PA1 Tsaur et al U.S. Pat. No. 5,171,659; PA1 Tsaur et al U.S. Pat. No. 5,210,013; PA1 Kim et al U.S. Pat. No. 5,272,048; PA1 Delton U.S. Pat. No. 5,310,644; PA1 Chang et al U.S. Pat. No. 5,314,793; PA1 Sutton et al U.S. Pat. No. 5,334,469; PA1 Black et al U.S. Pat. No. 5,334,495; PA1 Charfee et al U.S. Pat. No. 5,358,840; and PA1 Delton U.S. Pat. No. 5,372,927. PA1 each of R.sub.1, R.sub.2, R.sub.3 and R.sub.4 can independently represent an alkylene, cycloalkylene, alkarylene, aralkylene or heterocyclic arylene group or, taken together with the nitrogen atom to which they are attached, R.sub.1 and R.sub.2 or R.sub.3 and R.sub.4 complete a 5 to 7 member heterocyclic ring; and PA1 each of A.sub.1, A.sub.2, A.sub.3 and A.sub.4 can independently represent hydrogen or a radical comprising an acidic group, PA1 with the proviso that at least one A.sub.1 R.sub.1 to A.sub.4 R.sub.4 contains an acidic group bonded to the urea nitrogen through a carbon chain containing from 1 to 6 carbon atoms. PA1 X is preferably sulfur and A.sub.1 R.sub.1 to A.sub.4 R.sub.4 are preferably methyl or carboxymethyl, where the carboxy group can be in the acid or salt form. A specifically preferred tetrasubstituted thiourea sensitizer is 1,3-dicarboxymethyl-1,3-dimethylthiourea. PA1 L is a mesoionic compound; PA1 X is an anion; and PA1 L.sup.1 is a Lewis acid donor. PA1 R.sub.1 =(IVa) hydrogen or (IVb) alkyl or substituted alkyl or aryl or substituted aryl; and PA1 Y.sub.1 and Y.sub.2 individually represent hydrogen, alkyl groups or an aromatic nucleus or together represent the atoms necessary to complete an aromatic or alicyclic ring containing atoms selected from among carbon, oxygen, selenium, and nitrogen atoms. PA1 XI. Layers and layer arrangements PA1 XII. Features applicable only to color negative PA1 XIII. Features applicable only to color positive PA1 XIV. Scan facilitating features PA1 XV. Supports PA1 XVI. Exposure PA1 XVII. Physical development systems PA1 XVIII. Chemical development systems PA1 XIX. Development PA1 XX. Desilvering, washing, rinsing and stabilizing (post-development)
In addition to the collagen and ossein sought to be extracted in the preparation of gelatin there are, of course, other materials entrained. For example, James The Theory of the Photographic Process, 4th Ed., Macmillan, 1977, p. 51, states:
Superimposed on the complexity of composition is the variability of composition, attributable to the varied diets of the animals providing the starting materials. The most notorious example of this was provided by the forced suspension of manufacturing by the Eastman Dry Plate Company in 1882, ultimately attributed to a reduction in the sulfur content in a purchased batch of gelatin.
Considering the time, effort, complexity and expense involved in gelatin preparation, it is not surprising that research efforts have in the past been mounted to replace the gelatin used in photographic emulsions and other film layers. However, by 1970 any real expectation of finding a generally acceptable replacement for gelatin had been abandoned. A number of alternative materials have been identified as having peptizer utility, but none have found more than limited acceptance. Of these, cellulose derivatives are by far the most commonly named, although their use has been restricted by the insolubility of cellulosic materials and the extensive modifications required to provide peptizing utility.
Research Disclosure, Vol. 365, Sept. 1994, Item 36544, II. Vehicles, vehicle extenders, vehicle-like addenda and vehicle related addenda, A. Gelatin and hydrophilic colloid peptizers, paragraph (1) states:
This description is identical to that contained in Research Disclosure, Vol. 176, December 1978, Item 17643, IX. Vehicles and vehicle extenders, paragraph A. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power, both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be realized by increasing the proportions of selected high (&gt;50 mole %) bromide tabular grain populations in photographic emulsions.
In descriptions of these emulsions, as illustrated by Kofron et al U.S. Pat. No. 4,439,520, the vehicle disclosure of Research Disclosure Item 17643 was incorporated verbatim. Only gelatin peptizers were actually demonstrated in the Examples.
Despite the assumption that conventional vehicle selections are fully applicable to tabular grain emulsions, there have been some indications that some peptizer selections are particularly advantageous for tabular grain emulsions. Maskasky U.S. Pat. No. 4,400,463 disclosed the use of synthetic peptizers in combination with adenine to produce high (&gt;50 mole %) chloride tabular emulsions. Later Maskasky U.S. Pat. Nos. 4,713,320 and 4,713,323 demonstrated that high bromide and high chloride tabular grain emulsions could be improved by treating gelatin with an oxidizing agent.
Maskasky U.S. Pat. No. 5,284,744 taught the use of potato starch as a peptizer for the preparation of cubic grain silver halide emulsions, noting that potato starch has a lower absorption, compared to gelatin, in the wavelength region of from 200 to 400 nm. Maskasky '744 does not disclose tabular grain emulsions.